Thank you, Bob. I've at last got round to installing that plug-in.
However, although that web page says:
"It will show up in your File menu as Save-export clean, probably right under
Save a copy."
- here (on Linux Mageia-4) all I can see is "overwrite" (with the 'w'
underlined.

Overwrite is built into GIMP 2.8, not related to Save-export clean.
Perhaps you forgot to make the plug-in executable?
However: I've been meaning to announce a much more flexible save
plug-in, "Saver", which I've been using myself for nearly a year.
I've added a description of it and links to it on that same
gimp-save page Bob mentioned. Saver gives you the option of a
"Saver as..." dialog so you can save/export to different filenames,
and it also lets you save as XCF and at the same time, export a
JPG, possibly scaled down; I've found that useful when building
images for the web.
No warranty or guarantee implied, but for people who find themselves
bugged by the Save/Export split, this may be a workable solution.
...Akkana